The present invention relates to a method for the preparation of vitrified silica particles. More particularly, the invention relates to a method for the preparation of vitrified silica particles suitable as a base material of transparent fused silica glass bodies from extremely fine silica particles having no usefulness as such due to fluffiness to cause difficulties in handling.
As is known, synthetic fused silica glass is prepared by several different methods including the so-called direct method, soot method and sol-gel method in a gross classification. In the so-called direct method, a vaporizable silicon compound is introduced into an oxyhydrogen flame in which the silicon compound such as silicon tetrachloride is flame-hydrolyzed and converted into fine silica particles to be deposited on a rotating substrate body to form a porous silica body which is vitrified in situ as it grows by heating with the oxyhydrogen flame. This method is widely practiced in the preparation of fused silica glass bodies useful in the manufacture of various kinds of optical parts because the fused silica glass body prepared by this method has a high purity and is free from occlusion of bubbles.
In the soot method, on the other hand, the porous silica body formed by the deposition of the silica particles, i.e. silica soot, produced from a silicon compound in an oxyhydrogen flame on the rotating substrate is, without being vitrified in situ as it grows, vitrified in a separate electric furnace into a transparent silica glass body. Because the conditions for the flame hydrolysis of a silicon compound can be moderate as compared with the direct method, the soot method is practiced in the preparation of a preform as a precursor of optical fibers which must have a modified refractive index by doping with a refractive index-modifying dopant such as germanium and the like. Further, the porous silica glass body before sintering and vitrification can be subjected to a dehydroxylation treatment or doping with fluorine and other vaporizable dopants by utilizing the porosity thereof.
The sol-gel method is a wet process in which a silicate ester such as tetraethoxy silane is subjected to a hydrolysis-condensation reaction to form a block of semi-solid sol which is then dried and sintered at a high temperature into a vitrified transparent silica glass body. A problem in this method is that formation of cracks is sometimes unavoidable in the block of the semi-solid gel so that this method is hardly applicable to the preparation of a large silica glass body. Accordingly, this method is rarely utilized in the preparation of fused silica glass articles and the only way to industrially practice this method is for the preparation of a silica filler as a compounding material of molding resins and the like by pulverizing the sintered body.
A problem common in the direct and soot methods is the low yield in the deposition of the silica particles relative to the amount of the consumed vaporizable silicon compound. Namely, the yield in the direct method is usually about 30% and the yield in the soot method also can rarely exceed 50% with a great loss of the silicon compound due to the loss of the silica particles formed therefrom but dissipated or discharged out of the oxyhydrogen flame furnace without being deposited to form the porous silica body. The silica particles recovered in a filter bag and the like as a flue dust have a very small particle diameter of 0.2 .mu.m to several micrometers in the form of a very fluffy powder difficult to handle. The thus recovered silica powder as such has almost no industrial usefulness due to the uncontrolled particle size distribution and possible contamination and must be discarded or disposed as an industrial waste material sometimes to cause serious environmental pollution.
As a consequence of the above mentioned problem in the prior art methods for the preparation of fused silica glass in which a half amount or even larger of the starting silicon compound is consumed futilely, the cost for the preparation of synthetic fused silica glass is increased so much even if not to mention the problem in respect of the environmental pollution. Thus, it is an important problem in the technology for the preparation of synthetic fused silica glass to establish an efficient way for the effective utilization of the extremely fine fluffy silica particles produced in the process of flame hydrolysis of a vaporizable silicon compound and heretofore discarded as an only way for the disposal thereof.